A joint seal comprises two elastomeric elements, configured to fit as a nested assembly within the gap between adjacent slabs. The outer element (the "outer seal") is a generally U-shaped channel (with sides and a bottom), and performs the actual sealing function. The outer surfaces of the channel sides are formed with a series of longitudinally-extending fins, which frictionally engage the slab edges. The inner element ("the core") is formed as a generally rectangular tube that provides a structural strength for the seal assembly. It includes a tubular outer wall and an internal truss-type framework consisting of a number of interconnected webs. The seal assembly is installed under compression.